There are many different kinds of cancer. Most cancers form solid tumors, and these tumors usually start with a series of mutation in one of the body's own cells. These mutations allow the affected cells to start dividing uncontrollably, and often to avoid the body's normal defenses against them. Sometimes just the physical presence of the tumor itself is the biggest problem. On the heart or brain, for example, a big tumor can prevent the organ from functioning normally and can even cause death.

More often, however, what ends up killing the cancer patient is what's known as metastasis. This is when cells from a tumor separate from it, find their way into the lymph system or the bloodstream, and spread throughout the body. When this happens, the tumor is said to be malignant. (Benign tumors are those that do not spread. They can still cause problems in some cases, like the heart or brain.) Particular types of malignant tumors often "metastasize" to particular organs--for example, colon cancer tumors often metastasize to the liver. But cancer cells from malignant tumors can invade many different tissues, such as bone, lungs, spleen, and more.

Each metastatic cell begins dividing and forming a new tumor in its new location. This is where the real problem is. Our bodies usually can't support the growth of that many tumors, and the tumors can disrupt the normal function of the organs they're growing in. If that happens, and if the disease is left untreated, the patient will die. Treating a patient who has malignant cancer is difficult, because the metastatic cells are actually the patient's own cells! Chemotherapies are usually designed to kill all rapidly dividing cells, but some rapidly dividing cells are normal, as in hair follicles and the stomach lining. This is why people taking chemotherapy often lost their hair and become nauseated. So the goal is to kill enough rapidly dividing cells to kill the tumors, but not so many that the patient is killed.

There are several ways it might. First, cancer cells metabolize (i.e. eat) other cells in the body, thus causing tissue damage to whatever they happen to be growing in. Second, cancer cells physically get in the way of other cells just by being there and not doing the job they are supposed to. Last, all tissues have functions, and the function of a tissue is lost because it is composed of cancer cells, that can be very bad.

Cancer results when a cell that is not growing suddenly decides to grow out of control and form a tumor. The way in which cancer kills varies, depending on what is affected. The most lethal cancers are those that are metastatic, where a single cell breaks off, migrates and starts a new tumor. Basically the tumor can fill up an organ and preclude it from functioning. The following is from cancerhelp.org.uk

Generally speaking, if cancer spreads to take over a part of the body that performs an essential function this can kill you. For example, if the cancer is growing in part of the digestive system, it can prevent the digestion and absorption of food. It can grow so that it blocks the digestive system and food cannot go through the intestines. If food cannot pass through, then the nutrients from the food cannot be absorbed.

If cancer is affecting the lungs, then eventually there is too little effective lung tissue to allow enough oxygen to be absorbed into the body to sustain life. Or the cancer can block off part of the lung. This part then collapses and often becomes infected. A person with an advanced cancer does not always have the strength to fight off such an infection, even with the help of antibiotics and so the infection can eventually lead to death.

If the cancer has spread to the liver or the bones, this can upset the body's delicate chemical balance. The human body operates within very fine limits of certain body salts and chemicals. For example, there has to be a certain amount of calcium in the circulating blood. Too much or too little can upset the whole system. If the cancer is affecting many of the bones in the body, then a lot of calcium is released into the blood stream.

Normally the body has mechanisms to fight this sort of imbalance. But when the balance becomes too much out of control, then the mechanisms to correct the imbalance can become overwhelmed. There are treatments to control too much calcium, but these too can become overwhelmed and then unfortunately the calcium will continue to rise in the blood until the person affected becomes unconscious and eventually dies.

The liver is the chemical factory of the body. It performs many different functions mostly to do with maintaining the balance of body chemicals. When there is not enough healthy liver tissue to keep the balance, for example in very advanced liver disease, patients may become unconscious when the body's chemical balance becomes severely upset.

When cancer is growing in the bone marrow, then eventually there will not be enough healthy bone marrow to make blood cells. This will cause anemia (not enough red blood cells) and not enough oxygen will be carried around the body. It will also cause a drop in white blood cells. As these fight infection, it becomes more and more difficult for the body to keep bacteria and viruses under control. A drop in platelets will prevent the body from controlling any abnormal bleeding. If a blood vessel in a vital part of the body is damaged, for example in the brain, then the resulting bleeding can cause a stroke which is likely to be fatal as the body cannot control the bleeding.

Some cancers produce particular substances directly which will upset the body balance. This can cause problems such as severe weight loss or dehydration which will eventually overwhelm the natural corrective mechanisms the body has.

Many treatments can control cancer for a long time even if they can't provide a cure. But if a cancer continues to grow, then unfortunately it can become too much for the body to cope with and ultimately the treatment can no longer keep it at bay.

Although this is a difficult subject for people to talk about (including doctors and nurses), it may help you to ask your specialist doctor or nurse about how you or your relative may die. It is something most people worry about at some point and talking about the way the cancer may affect your body can help to lessen at least some of those worries. Many people are relieved to find out that they (or their relative) are likely to become unconscious shortly before they die. It can be far more worrying to bottle up your fears - what you imagine may happen before death is often far worse than what actually will.

It is important to remember also that very good pain control is available and no one with cancer should die in pain.

First, it is useful to have a working definition. "Cancer" is actually a collection of diseases that share some common features (uncontrolled cell growth being the most obvious). Clinicians define cancer as unregulated cell growth leading to invasion of surrounding tissues and spread (metastasis) to other parts of the body.It is not always obvious how cancer kills. Most frequently, it is due to a disruption in the function of a vital organ -- wherever there is uncontrolled cell growth, the "normal" cells of that organ may be impaired; either too many cells become cancerous and non function or the tumor cells deny proper nutrients to the remaining normal cells. This results in organ failure. But this does not explain, for example why bone metastases are so lethal. Typically, a cancer patient may suffer from "wasting" (called "cachexia") - this may be due to toxins that get released into the body - either from the tumor cells or in response to the tumor cells. There are many unanswered questions regarding causes of cancer and of course, prevention, treatment and cure. Just this week, it was announced that researchers had made a vaccine against a virus typically associated with cervical cancers! And as you probably know, most chemotherapy drugs are directed at fast dividing cells -- killing normal fast dividing cells as well as the cancer cells, leading to side effects. A new area of research, called cancer stem cell biology, offers some hope for new therapies based on the idea that tumors actually are derived from a slow dividing stem cell. I encourage you to check out the information on the websites at the National Institutes of Health and the American Cancer Society.

I used to ask myself this question, too. People often describe cancer as "uncontrolled cell growth". If cancer tumors really are just areas in the body where cells are growing out of check, how can this actually harm a person?

Technically, not all cancers are fatal. Many older men get prostate cancer but few die from it. (I think the actual number of men who will be diagnosed with prostate cancer before they die is around 17%; the number of men who will die from it is around 3%. These statistics are from The American Cancer Society's web page.) This is mostly because the symptoms of prostate cancer are hard to ignore (pain during urination, impotence) and early diagnosis and treatment are effective in slowing the growth and spread of this type of cancer.

Scientists used to think that cancer was one disease with one primary cause. This would be nice, because then finding a cure would be that much easier. Unfortunately, we now know that different cancers have different causes, so each cancer needs to be researched separately to find out the causes and most effective treatments. For this reason, we are nowhere near a cure for cancer in general, although we may be close for certain types of cancer.Cervical cancer in women has been linked to infection by the human papilloma virus (HPV) Just recently, a vaccine for HPV has been developed that may, in fact, cure cervical cancer if used effectively. Certain people in the government are fighting this, however.

Because all types of fatal cancers are different, the way in which they are fatal is different. Some of the most fatal cancers are those which grow in a part of the body that is essential for life: the lungs, the intestines, the brain, the liver, bone marrow. Cancer in essential organs of the body can cause these organs to fail. The tumor, or area of rapid new cell growth, is not functional and can crowd out nearby functional cells or rob them of blood and nutrients. Lung cancer can cause a lung to collapse, stomach or intestine cancer can block your digestion, brain cancer can build pressure in your brain or crowd out vital brain tissue, liver cancer can cause your liver to fail, and bone marrow cancer can impede your body's ability to make blood cells, which in turn impedes your body's ability to transport oxygen and fight off infection. Because the major organs are so vital for life, these are all parts of the body that are hard to operate on. Assuming the operation is successful and you remove the tumor without lasting damage to the organ, the tumor may grow back. All it takes is a few cells of the original tumor remaining in the body. This is why surgery combined with chemotherapy are important for treating these cancers. Bone marrow cancer, or leukemia, is hard to treat because you can't operate to physically remove the cancerous cells, so chemotherapy and a bone marrow transplant are often the only options.

But why would a tumor growing in breast tissue or the skin of your face pose a life-threatening problem? One answer is that cancerous cells can often spread from the original site of the tumor to other parts of the body via the blood stream and lymph system. So a tumor growing in breast tissue or the skin on your face may actually cause a tumor to develop in your brain or lungs if left unchecked. Certain types of cancerous cells grow faster and/or spread more aggressively than others. Also, tumors are areas of rapidly growing cells and need a lot of blood supply and nutrients, so they can rob nearby cells of nutrients and kill them, causing damage near the tumor site. Some tumors produce toxins that can affect how nearby cells function, or even cause massive changes in the entire body, such as dehydration or rapid weight loss.

The American Cancer Society has a web page with a lot of information on cancer: American Cancer Society